Centrifugal compressors used in a compressor part or the like of turbochargers in vehicles or ships give a kinetic energy to a fluid through rotation of a vaned wheel and discharge the fluid radially outward by the centrifugal force to raise the fluid pressure. In response to the demands for a high-pressure ratio and high efficiency in a wide operation range of such centrifugal compressors, impellers (vaned wheels) 05 having splitter blades 03 each arranged between full blades 01 adjoining each other as shown in FIG. 9 and FIG. 10 are commonly used.
Such impeller 05 with splitter blades 03 includes the full blades 01 and the splitter blades 03 arranged alternately on the surface of a hub 07. Common splitter blades 03 have the same shape as the full blades 01 with their upstream sides simply cut off.
The inlet edge (LE2) of the commonly known splitter blade 03 is located a preset distance downstream of the inlet edge (LE1) of the full blade 01 as shown in FIG. 11, while the trailing edges (TE) are placed at the same position. The blade angle θ at the inlet edge of the splitter blade 03 (indicated as an angle made between the direction of the inlet edge and the axial direction G of the impeller 05) is set the same as that of the flow direction F of the fluid flowing through the flow passage between the full blades 01.
Meanwhile, techniques of making the throat areas of two passages formed on both sides of each splitter blade 03 equal so as to distribute the fluid evenly have been known. Patent Document 1 (Japanese Patent Application Laid-open No. H10-213094), for example, discloses a technique in which, as shown in FIG. 12, the blade angle θ at the inlet edge of the splitter blade 09 is set larger to be θ+Δθ, (the angle is set larger by Δθ relative to the flow direction F of the fluid), i.e., the splitter blade is positioned closer to the suction side Sb of the full blade 01, in order to make the throat areas of the passages on both sides of the splitter blade 09 equal (A1=A2).
The positioning of the inlet end of the splitter blade inclined to the suction side of the full blade is also known from the disclosure in Patent Document 2 (Japanese Patent Publication No. 3876195).
Patent Document 1: Japanese Patent Application Laid-open No. H10-213094
Patent Document 2: Japanese Patent Publication No. 3876195
The techniques shown in Patent Documents 1 and 2 both relate to an improvement in the blade shape in respect of flow rate distribution in flow passages divided by the splitter blade based on an assumption that the fluid between the blades flows along the full blades. In open type impellers with a tip clearance, the flow field is complex due to the tip leakage flow coming into or out of the passage through the tip clearance, because of which a further improvement was needed to the blade shape to better adapt to such complex internal flow.
An evaluation of such complex internal flow through a numerical analysis revealed that the tip leakage vortex (vortex flow leaking at the blade tip as shown in FIG. 8, hereinafter referred to as “tip leakage vortex W”) generated from the tip of the inlet edge of the full blade (the distal end of the blade (on the shroud side) in the direction of height from the hub surface) reached the vicinity of the tip of the inlet edge of the splitter blade (the distal end of the blade (on the shroud side) in the direction of height from the hub surface).
In view of this, the present inventors filed a patent application (Japanese Patent Application No. 2009-233183, not published yet) relating to a technique of preventing the tip leakage vortex W from interfering with the splitter blade by inclining the leading edge of the splitter blade toward the suction side of the full blade.
To accomplish an even higher pressure ratio and efficiency, and a wider range of operation of the centrifugal compressor, it is essential to increase the number of blades. Providing two or more splitter blades is therefore a significant technique, but Patent Documents 1 and 2, or the previous application mentioned above, do not disclose any specific improvements in regard to a plurality of splitter blades.